1. Field of the Invention
The present invention relates to a noise reducer, a noise reducing method, and a computer program, which serve to reduce a noise by reducing a spectrum component of a noise signal from the spectrum component of the inputted signal in which the noise signal is superimposed on a speech signal.
2. Description of the Related Art
Due to development of a computer technology in recent years, a recognition accuracy of speech recognition has been rapidly improved. Then, in order to further improve the speech recognition accuracy, as preparation for the inputted speech, various noise reducers to reduce a noise including nonstationary noise such as speech and a musical composition other than a target of recognition by the audio processing have been improved.
FIG. 7 is a block diagram showing a constitutional example of a conventional noise reducer. As shown in FIG. 7, the conventional noise reducer is provided with a speech accepting part 701, a signal converting part 702, a noise reducing part 703, a signal restoring part 704, an amplitude calculating part 705, and a coefficient calculating part 706.
The speech accepting part 701 accepts input of speech. The signal converting part 702 converts a signal on a time axis of the inputted speech into a signal on a frequency axis. The amplitude calculating part 705 calculates the amplitude component of the signal on the frequency axis, and the coefficient calculating part 706 calculates a noise reduction coefficient.
In FIG. 7, the speech including the noise is accepted by the speech accepting part 701 to be converted into the signal on the frequency axis by the signal converting part 702. For example, in the signal converting part 702, time-frequency conversion processing such as a Fourier transform and a plurality of band pass filtering processing such as sub band decomposition processing or the like are carried out.
The signal on the frequency axis that is converted by the signal converting part 702 is multiplied by a coefficient due to the noise reducing part 703. The coefficient of the noise reducing part 703 is a noise reduction coefficient to be described later. For example, in a frequency band only containing a speech, a coefficient is defined as “1” and in the frequency band only containing noise, a coefficient is defined as “0” or a sufficiently small value.
The signal of which noise is reduced by the noise reducing part 703 is converted from the signal on the frequency axis into the signal on the time axis by the signal restoring part 704 to be outputted. The processing of the signal restoring part 704 is the inverse transformation of the signal converting part 702.
The signal on the frequency axis that is converted by the signal converting part 702 is also inputted to the amplitude calculating part 705. The amplitude calculating part 705 calculates the amplitude component of the inputted signal for each frequency band. The coefficient calculating part 706 extracts the amplitude component at the frequency band where only a noise exists on the basis of the amplitude component of the inputted signal that is calculated by the amplitude calculating part 705 by using the variation amounts or the like in the time axial direction of the inputted signal and calculates a noise reduction coefficient by using an amplitude component of a signal (a stationary noise signal) only including the extracted noise.
As described above, according to the conventional noise reducer, by assuming that there is no correlativity between the noise signal and the speech signal and estimating that the amplitude component at the frequency band where the noise only exists is the amplitude component of the stationary noise signal, the amplitude component of the noise is subtracted from the amplitude component of the inputted signal at each frequency band or by carrying out the level reduction equivalent to the subtraction, the noise is reduced.
In addition, according to the above-described noise reduction, the amplitude component of the noise is subtracted from the amplitude component of the inputted signal in excess, so that this involves a problem such that the speech signal and the remaining noise or the like are distorted. In other words, reduction of the speech signal and the noise or the like in excess generates a discontinuous point in the outputted signal and a friction sound, a so-called musical noise or the like is generated. In order to solve such a problem, for example, the noise reducer disclosed in Japanese Patent Application Laid-Open 2001-249676 is provided with a target value setting part 707 for setting a target value of reduction of the noise so as to prevent the speech signal from being distorted by only subtracting the amplitude component of the noise till this target value.